1. Field of Invention
The present invention relates to an electronic timepiece, a time adjustment method for an electronic timepiece, and a control program for an electronic timepiece that receives radio signals transmitted from positioning satellites such as GPS satellites and acquires the current date and time.
2. Description of Related Art
The Global Positioning System (GPS) for determining the position of a GPS receiver uses GPS satellites that circle the Earth on known orbits, and each GPS satellite has an atomic clock on board. Each GPS satellite therefore keeps the time (referred to below as the GPS time or satellite time information) with extremely high precision.
All GPS satellites transmit the same GPS time, and the Universal Coordinated Time (UTC) is acquired by adding the UTC offset (currently +14 seconds) to the GPS time. For an electronic timepiece to receive the satellite signal transmitted from a GPS satellite, acquire the GPS time, and display the local time (regional time) at the location where the electronic timepiece is being used, the time difference to the UTC must be added after correcting for the UTC offset in order to get the current local time, and the electronic timepiece must therefore know what this time difference is.
The UTC offset can be acquired from the data in the received satellite signal, or a predetermined value stored in ROM.
Japanese Unexamined Patent Appl. Pub. JP-A-H11-183594 is directed to a GPS device that gets the time difference information from location information after the user of the electronic timepiece selects the current location.
This GPS device stores information about selected major locations (such as country, city, or region name) around the world, including the time zone, latitude, longitude, and the geographic coordinate system, in memory. The user selects the location closest to the user's current location to set these parameters, which are then used by the GPS device to calculate and display the time at the current location based on the received GPS time.
Japanese Unexamined Patent Appl. Pub. JP-A-H08-68848 is directed to a GPS navigation system that acquires positioning information using satellite signals transmitted from the GPS satellites, and automatically calculates the time difference at the current location from the positioning information.
This system has a storage device that stores border information for converting the positioning data to time zone data.
Japanese Unexamined Patent Appl. Pub. JP-A-2003-139875 is directed to a radio-controlled timepiece that has a fixed-point data storage means that stores multiple fixed point data records containing fixed position information, fixed time difference information, and fixed range information, and automatically corrects the GPS time by acquiring the mobile device position using satellite signals from the GPS satellites, getting the fixed position information nearest the mobile device position, and using the fixed time difference for the fixed position as the time difference for the mobile device.
Problems with the related art described above are described below.
A problem with the GPS device in JP-A-H11-183594 is that because the list of selectable city names is displayed as a fixed list, selecting the current location can be difficult for the user depending on the location.
For example, if the city names are arranged in order of the time zone starting from Honolulu, where the time difference to UTC is −10 hours, Sydney, Australia and Wellington, NZ will be displayed as locations farthest from Honolulu. This requires the user to move the selection cursor a long distance from Honolulu, and making a selection is thus laborious.
The GPS navigation system in JP-A-H08-68848 automatically acquires the time difference based on the current location identified using the positioning signals and the previously stored border location data, and is thus convenient because it eliminates the need for the user to select the location as required with the GPS device in JP-A-H11-183594.
However, border data for all time zones around the world must be stored in the storage means to prevent detecting the wrong time difference. However, the borders between time zones are often winding national borders, and storing data for such complex borders requires a large storage capacity. As a result, size and cost constraints limit the data storage capacity in a wristwatch or other small portable device, and thus prevent being able to store such complex border data. JP-A-H11-183594 is therefore limited in the devices in which it can be used, and cannot be used in small devices such as wristwatches.
Addressing the large data storage requirement of JP-A-H11-183594, JP-A-2003-139875 provides a radio-controlled timepiece that only needs to store fixed point data for selected major cities around the world, and thus reduces the amount of data to be stored and the required storage capacity.
However, because the technology taught in JP-A-2003-139875 extracts the fixed position information closest to the mobile device position by selecting a circular area centered on a particular fixed position and uses the time difference for that fixed position if the position of the mobile device is in range of the selected fixed position, the possibility of selecting the wrong fixed position and therefore the wrong time zone (time difference) is high in areas where the time zone borders overlap.
In order to adjust the size of these circular areas, distances are normalized using a weighting coefficient referred to as the fixed range information. However, when the time zone borders overlap and there are multiple fixed positions around and near the location of the mobile device, it is difficult to set the fixed ranges so that detection errors do not occur, and the amount of data that must be stored increases.
Furthermore, because the distance between the mobile device and each fixed position must be calculated, computation time increases when there are multiple fixed positions in the vicinity of the mobile device, the time difference cannot be set quickly, and convenience is thus impaired.